The invention originates in the field of data transmission. It has to do in particular with transmitting real-time data in combination with less time-critical data within one transmission period.
In a communications system, data telegrams are periodically exchanged between stations. Besides this so-called cyclical communication, which is preferably done in real time, there is a need for so-called unplanned or acyclical communication, in which arbitrary protocols may be incorporated within a time slot that is binding for all the participants.
European Patent Disclosure EP 1 312 992 discloses a method for transmitting a higher-level protocol such as TCP/IP without influencing the cyclical data traffic by providing that a subordinate device (slave), by means of an alarm request, initiates the transmission and causes an overriding device (master) to call up further message data sets in the subordinate device by means of an identification data field. The master, using a destination address furnished by the slave, forwards the message data sets on to another slave.
US Patent Disclosure 2002/0091838 shows a method for adapting the data rate in a field bus so that an increasing quantity of data can be handled. A new transmission rate is agreed upon by all the bus participants and used by them for further communication. Upon startup of operation, a new calibration is made among all the participants.
During initial test constructions by the present applicant, it has been found that depending on the participant-specific quantity of data in the real-time channel, situations can occur in which, because the bandwidth is too narrow, required additional data can no longer be transmitted if their telegram length becomes too great. This problem occurs especially when bus participants are turned dynamically on and off. On the assumption that at a transmission rate of 100 MBit/s, for example, a telegram duration of 1500 bytes is intended for acyclical additional data, a time slot required for this would be 1500×8 bits/byte/100 MBit/s=120 μs. On the assumption that a transmission cycle lasts 128 μs, only 8 μs would thus be available for the cyclical real-time data, which in practice would likely be too short.